We are creating computer models of early C. elegans embryos to identify blastomeres that contact one another and are therefore candidates for cell signaling events early in animal development. Studies of embryogenesis provide evidence that specific cell-cell contacts are required for the correct induction of pharyngeal, neuronal, and hypodermal tissues (Hutter and Schnabel, 1994; 1995; Moskowitz et al., 1994). We are using images of 12-blastomere stage embryos obtained by high voltage electron microscopy of 0.25 fm sections and time lapse light microscopy with Nomarski optics to generate three dimensional computer models. Electron microscopy enables us to locate accurately the cellular membranes in the embryo, while time-lapse Nomarski microscopy permits the examination of larger numbers of animals and the unamibiguous identification of each blastomere. In addition to elucidating cell-cell contacts during embryogenesis, the computer models provide information about approximate cellular volume and surface area of each cell-cell contact. Other planned invesitagations, including an analysis of cell rearrangements during nematode gastrulation, are awaiting completion of IMOD software modifications currently in progress. Hutter, H. and R. Schnabel. (1994) glp-1 and inductions establishing embryonic axes in C. elegans. Development 120:(7):2051-2064. Hutter, H. and R. Schnabel. (1995) Specification of anterior-posterior differences within the AB lineage in the C. elegans embryo. a polarising induction. Development 121(5):1559-1568. Moskowitz, I.P.G., S.B. Gendreau, and J.H. Rothman. (1994) Combinatiorial specification of blastomere identity by glp-1-dependent cellular interactions in the nematode Caenorhabditis elegans. Development 120(11):3325-3338.